In manufacture of semiconductor devices such as typical LSIs (Large Scale Integrated circuits), photolithography technology is necessarily used to pattern insulating films such as silicon oxide and silicon nitride films formed on a semiconductor substrate into desired shapes. It is also used to pattern conductive films such as aluminum alloy and copper alloy films and a work itself including the semiconductor substrate.
In the photolithography, a UV-sensitive photoresist is applied on the work to form a photoresist film, to which a UV ray is radiated (which is exposed to a UV ray) through a mask pattern to turn a UV-radiated region into a soluble one (positive type) or an insoluble one (negative type). The photoresist film is then subjected to a developing process to partially remove the soluble one with a solvent for forming a resist pattern. Then, using the resist pattern as a mask, the work is selectively etched for patterning.
As an LSI is required to have a higher degree of functionality and performance, it is carried out to achieve a higher concentration and integration. Therefore, a need for photolithography technology to form fine circuit patterns becomes stricter. As a means for fine patterning, it is known to shorten a wavelength of a lithographic source for generating an exposing ray of light. For example, in mass production of a DRAM (Dynamic Random Access Memory) of 256 M bits through 1 G bits (its process size ranging from 0.25 μm to 0.15 μm), a far UV ray consisting of a KrF excimer laser ray of shorter wavelength (wavelength: 248 nm) is used instead of a UV ray consisting of an i-ray of the conventional type (wavelength: 365 nm).
In manufacture of a DRAM with an integration density of 4 G bits or greater (its process size being equal to 0.15 μm or less) that requires a fine pattern technology, a light source is required to radiate a far UV ray with a much shorter wavelength. In such a case, it is considered effective to use an ArF excimer laser ray (wavelength: 193 nm) and an F2 excimer laser ray (wavelength: 157 nm) in photolithography.
In particular, the photolithography using an ArF excimer laser ray (ArF excimer laser lithography) is an effective candidate for the next-generation fine patterning technology following to the KrF excimer laser lithography and is now increasingly studied. For example, Takechi et al., Journal of Photopolymer Science and Technology, vol. 5, No. 3, pp. 439-446 (1992); R. D. Allen et al, Journal of Photopolymer Science and Technology, vol. 8, No. 4, pp. 623-636 (1995) and vol. 9, No. 3, pp. 465-474 (1996).
In addition to a high resolution corresponding to the micro-patterned process size, a high sensitivity is required for the resists for lithography using the above-mentioned ArF and F2 excimer lasers, due to background situations such as a gas for use as a raw material in the laser oscillation having a short lifetime, an expensive lens being necessary, and the lens being able to be damaged easily by the laser. High sensitive photoresists suitable for such needs include a well-known chemically amplified resist that utilizes a photoacid generator as a photosensitive agent. The chemically amplified resist has a characteristic that allows the photoacid generator contained therein to generate a protonic acid due to light radiation. The protonic acid causes an acid catalytic reaction with a base resin and so forth in the resist by heating treatment after exposure. As a result, an extremely high sensitivity is achieved compared to that of a conventional resist that has a photoreaction efficiency rate (a reaction number per photon) less than 1.
As a typically known example of the chemically amplified resist, JP 2-27660A publication discloses a resist consisting of a combination of triphenylsulfonium hexafluoroarsenate and poly(p-tert-butoxycarbonyloxy-α-methylstyrene). Most of currently developed resists are of the chemically amplified type and thus development of high sensitive materials corresponding to shortened wavelengths of exposing sources is essentially required.
The above-mentioned chemically amplified resists used are classified into the positive and negative types. Among those, the chemically amplified resists of the positive type comprise at least three components: (1) a photoacid generator; (2) a base resin containing a group decomposable with acids; and (3) a solvent. On the other hand, the chemically amplified resists of the negative type are classified into two: one that essentially requires a crosslinking agent; and the other that requires no crosslinking agent. The former comprises at least four components: (1) a photoacid generator; (2) a base resin capable of reacting with a crosslinking agent; (3) a crosslinking agent; and (4) a solvent. The latter comprises at least three components: (1) a photoacid generator; (2) a base resin containing a crosslinking group; and (3) a solvent.
Examples of the photoacid generator that serves an important role in such chemically amplified resists include triphenylsulfonium salt derivatives as described in Journal of the Organic Chemistry, vol.43 (No. 15), pp. 3055-3058 (1978); alkylsulfonuim salt derivatives such as cyclohexylmethyl(2-oxocyclohexyl)sulfonium trifluoromethanesulfonate as disclosed in JP 7-28237A publication; and diphenyliodonium salt derivatives and succinimide derivatives as described in Journal of the Polymer Science, vol. 56, pp. 383-395 (1976).
Particularly, in the ArF excimer laser lithography, the most widely used photoacid generators at present are sulfonium salt compounds. Among those, triphenylsulfonium salt derivatives are most widely used currently. For example, see Nozaki et al., Journal of Photopolymer Science and Technology, vol.10 (No.4), pp.545-550(1997); and Yamachika et al.,Journal of Photopolymer Science and Technology, vol. 12 (No. 4), pp. 553-560 (1999).
One of important technical subjects on the resists for use in the lithography that uses a short-wavelength exposing source, represented by the ArF excimer laser, is to provide improved transparency to the exposing light of ray. This is because poor transparency lowers the resolution of the resist and worsens the pattern shape with trailing edges.
From such a viewpoint, unfortunately, though the above-mentioned triphenylsulfonium salt derivative is most widely used at present in the ArF excimer laser lithography as the photoacid generator consisting of a sulfonium salt compound, it has a disadvantage because its transparency is poor. That is, the triphenylsulfonium salt derivative has a benzene ring and thus strongly absorbs far UV rays not greater than 220 nm such as the ArF excimer laser characteristically. Accordingly, the triphenylsulfonium salt derivative is used as the photoacid generator to cause lower transparency of the resist. For example, see Naitoh Takuya, The 8th Research Group on Polymers for Microelectronics and Photonics, Proceedings, pp. 16-18 (1999).
Therefore, alkylsulfonium salt 2-oxocyclohexyl-methyl(2-norbornyl)sulfonium trifluoro methanesulfonate (NEALS), and cyclohexylmethyl(2-oxocyclo hexyl)sulfonium trifluoromethanesulfonate (ALS) are finally developed as new photoacid generators that are highly transparent against the ArF excimer laser. For example, see Proceeding of SPIE, vol. 2195, pp. 194-204 (1994); and Proceeding of SPIE, vol. 2438, pp. 433-444 (1995).
Though the above-mentioned newly developed photoacid generators consisting of sulfonium salt derivatives such as NEALS and ALS can improve transparency, they have disadvantages in sensitivity and thermal stability.
As for the sensitivity, in the ArF excimer laser lithography, a rate of sensitivity (exposure amount) of 20 mJ/cm2 and below (ideally of 10 mJ/cm2 and below) is required in general, though the above-mentioned NEALS requires an exposure amount of 50 mJ/cm2 and above. Therefore, lowered sensitivity can not be avoided. On the other hand, as for the thermal stability, a thermal decomposition point in a resist film (resinous film) is about 120° C. and below, the upper temperature at the steps of heating during film formation of and after exposure to the resist is limited to about 120° C. In the resists in which the above-mentioned sulfonium salt derivatives are used as the photoacid generators, a process of heating at about 125° C. and above is required to release an acid even from unexposed parts by decomposition. This heating is impossible and their thermal stability lowers.